Written by Nani Pradeepan, P.Eng.
We know that solar panels produce direct current (DC) electricity and our utility grid is powered by alternating current electricity (AC). Inverter used in solar power system converts solar DC electricity to AC electricity we all use at homes and other buildings.
As solar panels are rated at standard test conditions and not all installation locations and situations come close to these test conditions. Hence it often makes sense to oversize a solar array relative to inverter rating. This enable for greater AC electricity harvest when production is below the inverter’s rating, which is typically for most of the day and also can results in clipping.
Every inverter has a maximum rated power and it will generally never output more than their max-rated AC power. This is very important as the component ratings of the power electronics in the inverter are designed with a specific power and voltage range and the grid connection points in the buildings are designed with a specific max power limits.
When the DC solar array is generating the most amount of energy that is greater than the inverter’s power rating, the “extra” power generated by the array is “clipped” by the inverter to ensure it is operating within its capabilities. This leads to a flat line in the green curve in the figure below, and capping the power at the inverter’s nameplate power rating during peak production hours and known as inverter clipping.
The purple line in the figure shows a typical bell curve of AC output power peaking at noon, just below the rating of the inverter indicated by the dashed line. If we increase the size of the solar array by adding more panels, which increase the DC-to-AC ratio of the system (as illustrated by the green curve), we can harvest more energy throughout the day. The area between the green and purple curves is the energy that is gained by increasing the DC-to-AC ratio.
Standard test condition temperature is 25ºC but solar panels are hotter than 25ºC when the array is receiving maximum sunlight. Higher than rated temperature of 25ºC results in loss of power production when solar panel receive maximum sunlight. In other words the solar array produces less than the rated power when solar array receive maximum sunlight, this reduce the losses due to clipping of inverter.